The present invention relates generally to an injection molding apparatus and process of injection molding utilizing a novel control system which allows dynamic parameters of the injection process to be more accurately and easily controlled and which enhances the flexibility of operation of the apparatus. More particularly, the invention is directed to an injection molding apparatus and process of injection molding which utilizes a programmable logic controller to control certain variable dynamic parameters encountered in an injection molding process such as for example, the making intricate disposable wax patterns or the like for use in metal casting techniques.
In various injection molding procedures, it has been found that control over the injection process is less than adequate, resulting in defective molding which renders the procedure less efficient and adds to the cost thereof. It is desired in injection molding techniques such as die casting, plastic injection and wax injection techniques to provide a greater amount of control and flexibility in the injection process to enable more efficient, cost-effective production and manufacturing capabilities. For example, in metal casting techniques, such as investment casting, disposable wax patterns are utilized to generate a mold form used in the metal casting process. A molten metal will be poured into the mold form to produce metal castings which are often intricate in their geometry and require close tolerances with respect to characteristics such as dimension and surface finish. In the investment casting process, a disposable wax pattern is formed and thereafter covered with one or more layers of a suitable "investment" materials, such as a ceramic material, wherein the coating is permitted to solidify about the wax pattern. The wax pattern may then be removed by melting or dissolving the wax to leave a mold cavity in the investment material, being an exact replica of the wax pattern and having the desired intricate geometry and close tolerances of the metal part to be cast therein. Conventionally, such disposable wax patterns are produced by injecting a specially formulated liquid wax into a master mold using an injection molding apparatus, wherein the injected wax will be solidified under pressure within the master mold by means of the injection molding apparatus.
The intricate disposable wax patterns for use in investment casting or other metal casting techniques must be of high quality in order to be used to generate a suitable mold cavity as described. Similarly, cost effective production of such disposable wax patterns requires a high degree of uniformity and reduction of scrap patterns produced in the injection molding apparatus. As the intricate metal parts to be cast are configured in a broad range of size, geometry and other specifications, the master molds utilized for production of the disposable wax patterns similarly vary over a wide range. The broad variations in the size, geometry, orientation and runner systems of the various master molds require that the injection molding apparatus be capable of a great amount of flexibility, so as to be applicable for use in fabricating the desired wax pattern. The quality and uniformity of wax patterns produced in this manner are directly related to the ability of the injection molding apparatus to accurately control parameters of the pattern molding process. For example, the dynamics of fluid flow and pressure must be controlled with a high degree of accuracy to produce acceptable and desired wax patterns. As a wide variety of molds are to be used in the fabrication of wax patterns, a similar wide variety of wax flow characteristics and injection pressure control must be achievable in the wax injection molding apparatus. Similar criteria exist for various die casting, plastic injection and other injection molding techniques.
It is also been found that in many instances the final metal part to be cast includes hollow portions, requiring the wax pattern to be molded about a fragile ceramic core to enable production of such hollow castings. In such instances, the lack of adequate control of wax flow and pressure may result in breakage and/or structural damage of the fragile ceramic core thereby producing an unacceptable wax pattern. Further, in the wax injection process or any other injection molding process, the lack of adequate control in the injection process may also result in such imperfections as air bubbles, wax flow lines, knit lines, cracking or fracturing, incomplete fill, sink, incorrect size or dimensions and various other surface imperfections which may result in unusable patterns.
Various injection molding apparatus have been developed, as for example for use in fabrication of expendable wax patterns used in investment casting techniques, such as the assignees prior U.S. Pat. No. 4,274,823. The injection molding apparatus shown in this patent included a control system which allowed a high degree of control to be obtained over injection parameters such as the acceleration, maximum flow velocity and maximum pressure of the liquid wax with a high degree of accuracy throughout the injection and solidification cycle. A servo-control system was described which was operable to continuously monitor and variably control dynamic parameters of the injection molding cycle to achieve a high degree of quality and uniformity of the produced wax patterns. Although the wax injection molding apparatus greatly improved with the ability to properly fabricate desired disposable wax pattern for use in metal casting techniques in a cost effective manner, the apparatus comprised a dedicated system, wherein limitations for application of the system have been encountered. Under many circumstances, the wax injection molding apparatus of this prior patent was set up to act as a stand-alone dedicated machine capable of fabricating a single selected pattern with a high degree of quality and uniformity. It has been found that the basic inability to customize the wax injection molding apparatus to various needs of the user simply and effectively resulted in the need to provide a separate injection molding apparatus for each master mold to be used. It has also been found with the prior art injection apparatus, that the operator of the apparatus had the ability to modify the injection parameters to some degree, which may have resulted in less than adequate fabrication of the patterns if the operator was not extremely knowledgeable about the injection process. It has also be found to be desirable to provide the user with the ability to monitor operation of the injection molding apparatus and provide feedback to the user to allow optimization of the pattern fabrication process.